A resistance change type memory is known as a kind of semiconductor memory device. Also, as a kind of the resistance change type memory, an MRAM (magnetoresistive random access memory) is known. The MRAM is a memory device using a magnetic element having a magnetoresistive effect for a memory cell that stores information. Writing methods of the MRAM include a magnetic writing method and a spin injection writing method. Among these methods, the spin injection writing method has a property that a spin injection current needed for magnetization reversal decreases with a decreasing size of a magnetic material, making the method advantageous for higher integration, lower power consumption, and higher performance.
An MTJ (Magnetic Tunnel Junction) element as a memory element of the MRAM has a laminated structure including two ferromagnetic layers and a non-magnetic barrier layer sandwiched therebetween and stores digital data based on change of magnetic resistance due to a spin polarization tunnel effect. An MTJ element can take a low-resistance state and a high-resistance state depending on a magnetization relationship of the two ferromagnetic layers.
If the rate of resistance change (MR ratio) is small between the low-resistance state and the high-resistance state, it is difficult to read data correctly from an MTJ element.